Relativistic effects for NMR shielding constants in transition metal oxides using the zeroth-order regular approximation

Relativistic effects for NMR shielding constants have been calculated using the zero order regular approximation (ZORA) for relativistic effects. Isotropic NMR shielding constants were obtained using density functional theory with gauge including atomic orbitals (GIAO) in a spin-fi ec formalism for the metal nuclei in transition metal oxides MO4n- (M = Cr, Mn, Fe, Mo, Tc, Ru, W, Re, Os) and carbonyl complexes M(CO)(6) (M = Cr, Mo, W). The ZORA isotropic shieldings are compared with results from an extended version of the relativistic method employing the Pauli Hamiltonian developed earlier by Schreckenbach and Ziegler. Comparison between ZORA and Pauli shieldings, employing the restrictions necessary for the Pauli approach-frozen cores, restricted basis sets in the core region-show the ZORA shieldings to be significantly different from Pauli ones, but the chemical shifts of the metal oxides with respect to the carbonyl complexes do not differ much. However, extending the ZORA calculations (no frozen core, extended basis sets) gives significant changes, proving the limitation to frozen cores and restricted basis sets of the Pauli method to be unwarranted. Comparison to experiment shows that the errors of the most precise ZORA chemical shifts are ca. 10% (in the order of a few 100 ppm) for both the light and heavy transition metals. Error sources may be, apart from deficiencies of the density functional, the neglect of spin-orbit coupling and the neglect of solvent effects.